Photovoltaic, part 1

This is the first in a series of blog posts about the photovoltaic system (solar panels) we have on our house. We installed our system in Fall, 2006 and are enthusiastic about it. This first posting talks about the planning of the system; future posts will get into more of the details and the results we have gotten from it.

Motivations

First you need to think about why you want a photovoltaic system. Some people are motivated economically, and are looking for a reasonably short payback on the cost of the system. Others are doing this because it seems like the right thing to do, and want the satisfaction of doing something good for the environment, as well as some economic benefit from reducing or eliminating their electric bill and from increasing the value of their homes. We fit into the latter category.

When we started looking into this in 2006, the home solar installation and contracting industry was just emerging. There are now many options that didn’t exist then, especially for those whose motivation is economic. For example, some firms are leasing solar panels to homeowners to eliminate the capital outlay obstacle.

If you want to achieve a relatively short payback time for your system, you probably won’t be driving your electricity bill all the way to zero. That’s because electricity is often sold in a tiered pricing system: there’s usually a baseline number of kilowatt-hours (kWh) per month that are fairly cheap, and electricity beyond that is more expensive. With a smaller solar system, you can offset the more expensive power cost.

At least in California, it isn’t possible to get the utility company to pay you for extra electricity you generate; homes are considered “incidental” generators of power. Therefore, it probably doesn’t make sense to put up more solar panels than needed. However, they do average the bill for those with photovoltaic systems over an entire year, because it is expected that customers will be net consumers of power in the winter and net producers in the summer. This lets you take advantage of the extra power you produce in the summer when you need it in the winter.

Time of Use Metering

Depending on the power tariffs where you live, there may be a time of use metering plan available. This is a really good deal for people with photovoltaic. Why? The times when power is most expensive (typically weekday afternoons in the summer) are the times when photovoltaic systems generate the most power.

The time-of-use plan we are on is known as E-7, which designates a peak period between 12 noon and 6 pm on weekdays. This rate schedule has been replaced with E-6, which has three tiers in the summer and two tiers in the winter. E-7, which is no longer available to new customers, is generally considered to be more favorable to solar installations, although I haven’t done an analysis to see if that is actually the case.

Aesthetics

Since our house faces south, we were concerned about the visual impact of a photovoltaic installation.

There are several different types of photovoltaic collectors, including slate tiles, flexible shingles, and solar panels. While we looked to some extent at all three, it was clear that because of our wooden shake roof, we’d need to basically replace the roof in order to consider the first two. It also appeared that the solar panels were more efficient per unit area; so the fact that they’re more visible is somewhat mitigated by the fact that there is less area for a given power output.

We identified an area over the garage that is relatively less visible from the street than other areas of the house. We also came upon the idea of putting half of the panels on a west-sloping roof in the rear of the house. The contractor warned us that this would be about 15% less efficient than a front-facing panel, and this is confirmed by our data. However, since the time-of-use plan favors the afternoon hours, a west-facing array is more favorable than that from a financial standpoint.

Finding a contractor

Most of what we learned about finding a contractor is probably obsolete now. We talked with a few companies who had the right intentions, but didn’t follow through and left us questioning their ability to manage the project. A friend recommended Palo Alto Solar, whom we ultimately contracted with. During the project, Palo Alto Solar was acquired by SolarCity, which has since been actively marketing photovoltaic installation in the local market. A lot has changed in three years.

Determining what you need

We provided our contractor with copies of a year’s worth of electric bills, and they figured out what we needed to achieve our objective of driving the bill to zero. We then worked with them to plan the placement of the panels on the roof. You also need inverters to change the DC output of the panels to AC, synchronized with the power line. Having two arrays in different directions, we have two inverters to get the most efficiency out of the installation. The contractor’s estimate was very good; we generate only slightly more (in terms of cost) than we consume, and part of that is probably due to subsequent power conservation.

Power outages aren’t a significant problem where we are, so we didn’t consider any form of energy storage such as batteries. The cost of batteries would have been high for limited benefit, and also environmentally unfriendly in situations where they aren’t really needed.